In prior art air pressured powered hydraulic fluid pumps, the pumps may include a ball type valve as a throttling device for the pumps and for permitting the operator to control the quantity of air supplied to the hydraulic pump. One of the disadvantages of these arrangements is that the operator does not know how fast the air pump is turning and cannot properly adjust the valve. Even if a tachometer were employed to set the valve for low pressure operation of the pump, this control arrangement would require resetting in order to obtain optimum performance at higher hydraulic fluid pressures. The required constant adjustment of this arrangement makes it impractical.
In other prior art arrangements, an air regulator is used to limit the maximum air pressure supplied to the air pressure pump. This arrangement has deficiencies similar to those of the ball valve arrangement in that at higher hydraulic pressures, much of the pump performance will be lost.
Attention is also directed to the Houle U.S. Pat. No. 2,861,519, issued Nov. 25, 1958; the Credle U.S. Pat. No. 4,544,328, issued Oct. 1, 1985; the O'Neill U.S. Pat. No. 3,816,025, issued June 11, 1974; and the Henkel U.S. Pat. No. 2,630,758, issued Mar. 10, 1953.
Attention is also directed to the Moore U.S. Pat. No. 3,590,839 issued July 6, 1971; the Curnow U.S. Pat. No. 3, 787,027, issued Jan. 22, 1974; the Ashbrook U.S. Pat. No. 2,928,646, issued Mar. 15, 1960; the Cassarino U.S. Pat. No. 2,976,880, issued Mar. 28, 1961; the Hoffman U.S. Pat. No. 3,359,868, issued Dec. 26, 1967; the Pelton U.S. Pat. No. 2,509,880, issued May 30, 1950; the Bobnar U.S. Pat. No. 4,172,582, issued Oct. 30, 1979; and the Coppola U.S. Pat. No. 4,040,600, issued Aug. 9, 1977.